1. Field of the Invention
The present invention relates to a control system for an aircraft engine and, more specifically, it relates to a control system for a turbo-charged diesel engine equipped with a boost compensator.
2. Description of the Related Art
An aircraft equipped with a propeller driven by an internal combustion engine is well known in the art. For example, U.S. Pat. No. 5,810,560 discloses an aircraft equipped with a controllable pitch propeller driven by a gasoline engine. In the aircraft in U.S. Pat. No. 5,810,560, the set speed of propeller governor and the set amount of fuel injection is controlled by a single control lever. In U.S. Pat. No. 5,810,560, as a gasoline engine is used, the engine output is controlled by adjusting the degree of opening of a throttle valve disposed on the air intake passage of the engine, and the propeller governor and the throttle valve are connected to a single control lever by means of link and cam mechanism, and the set speed of the propeller governor and the degree of opening of the throttle valve change simultaneously in accordance with the position of the control lever. Therefore, the engine speed and the engine output power are controlled by a single lever in order to reduce complexity in the control of the aircraft.
A gasoline engine is used in the aircraft in U.S. Pat. No. 5,810,560. When a turbo-charged diesel engine is used for an aircraft, some problems occur.
In a gasoline engine, the engine output power is controlled by the degree of opening of the throttle valve. More specifically, the amount of engine intake air is determined by degree of opening of the throttle valve, and the amount of fuel supplied to the engine is automatically controlled so that combustion air-fuel ratio of the engine is kept at a predetermined value.
Therefore, the combustion air-fuel ratio does not change even if the engine output power is changed in a gasoline engine.
However, in a turbo-charged diesel engine, the intake air amount of the engine is not controlled by a throttle valve and is determined by the engine speed and the intake air pressure (i.e., the boost pressure). The engine output power is controlled by changing the amount of fuel injection. Therefore, a combustion air-fuel ratio changes in output power (the fuel injection amount). Thus, in some operating conditions, if the combustion air-fuel ratio becomes excessively low, exhaust smoke will be formed due to a shortage of combustion air.
Although the engine speed (the propeller speed) and the engine output power are controlled by a single lever in U.S. Pat. No. 5,810,560, even if the propeller speed and the engine output power are controlled separately by a speed lever and a throttle lever, this exhaust smoke problem also occurs.
In order to prevent the formation of exhaust smoke, a boost compensator is used in some turbo-charged diesel engines. The boost compensator is an apparatus that restricts the fuel injection amount to a value less than a maximum limit (i.e., so called “a smoke limit”) corresponding to an allowable lowest air-fuel ratio. The allowable lowest air-fuel ratio is a lowest air-fuel ratio on which the engine can operate without forming exhaust smoke. As the amount of air charged into cylinders of the engine increases as the boost pressure increases, the maximum limit of the fuel injection amount is determined by the boost pressure.
If a boost compensator is used for an aircraft turbo-charged diesel engine, as the combustion air-fuel ratio is controlled at a value lower than the smoke limit, an exhaust smoke problem does not occur.
However, if a boost compensator is used for an aircraft turbo-charged diesel engine, another problem occurs. In the aircraft operation, in some cases, it is necessary to increase the engine output power rapidly. For example, during a landing of the aircraft, if it is required to abort a landing operation and to raise the aircraft to a certain altitude (i.e., if so called a “go-around” is required), a rapid increase in engine output power is required.
In this case, however, if the engine is equipped with a boost compensator, it is difficult to increase the engine output power rapidly. If a boost compensator is used, the maximum amount of fuel supplied to the engine is restricted by the smoke limit regardless of, for example, the position of a throttle lever. As the smoke limit is determined by the boost pressure of the engine, the rate of increase in the amount of fuel supplied to the engine is restricted by the rate of increase in the boost pressure. Therefore, even if the throttle lever is operated rapidly in order to rapidly increase the engine output power, the actual engine output power, i.e., the actual fuel supply amount does not increase largely until the boost pressure increases.
Therefore, if a boost compensator is used, it is difficult to rapidly increase the engine output power.
It is true that, if the maximum limit of the fuel supply set by the boost compensator is set at a value larger than the smoke limit, the problem in increasing the engine output power may be solved. However, if the maximum limit of the boost compensator is set at a value larger than the smoke limit, the formation of exhaust smoke may occur even in a steady operation of the engine.